Abstract: The characteristics of magnetic field generated after the magnetization of pipeline were numerically simulated with finite element method based on magnetic flux leakage（MFL） principle. The ANSYS finite element software was used to perform the simulation for misalignment,undercut and pit defects in pipeline girth weld for L415 pipeline steel with wall thickness of 9 mm and diameter of 529 mm. The magnetic flux density distribution curves describing the features of magnetic leakage field were obtained,which was consistent with experiments. By changing the size of defects,the distribution rule of leakage magnetic field for different defect forms and sizes could be observed after comparative analysis. It provided theoretical basis and practical reference for characteristics identification of pipeline defects magnetic flux leakage signal.